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The conversion of vitamin D, especially cholecalciferol, to 25(OH)D (calcifediol) is one of the key steps in the vitamin D hormonal system. The CYP2R1 enzymatic activity achieving this process was previously thought to be constitutively expressed and stable, so that serum 25(OH)D was a measure of the supply of vitamin D. [9]
CYP24A1 also is able to catalyze another pathway which starts with 23-hydroxylation of 1,25-(OH) 2 D 3 and culminates in 1,25-(OH) 2 D 3-26,23-lactone. [ 6 ] The side chains of the ergocalciferol (vitamin D 2 ) derivatives, 25-OH-D 2 and 1,25-(OH) 2 D 2 , are also hydroxylated by CYP24A1.
Several forms of vitamin D exist, with the two major forms being vitamin D 2 or ergocalciferol, and vitamin D 3 or cholecalciferol. [1]The term 'vitamin D' refers to either D 2 or D 3, or both, and is known collectively as calciferol.
24,25-Dihydroxycholecalciferol, also known as 24,25-dihydroxyvitamin D 3 and (24R)-hydroxycalcidiol (abbreviated as 24(R),25-(OH) 2 D 3), [1] is a compound which is closely related to 1,25-dihydroxyvitamin D 3, the active form of vitamin D 3. Like vitamin D 3 itself and calcifediol (25-hydroxyvitamin D 3), it is inactive as a hormone both in ...
The natural, active form of vitamin D is calcitriol (1,25-dihydroxycholecalciferol). This molecule and other naturally occurring forms of vitamin D, including its precursors and metabolites, have been modified to synthesize pharmaceuticals with potentially greater, or selective, therapeutic actions. [1] [2] [3] [4]
7-Dehydrocholesterol (7-DHC) is a zoosterol that functions in the serum as a cholesterol precursor, and is photochemically converted to vitamin D 3 in the skin, therefore functioning as provitamin-D 3. The presence of this compound in human skin enables humans to manufacture vitamin D 3 (cholecalciferol).
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Calcifediol binds in the blood to vitamin D-binding protein (also known as gc-globulin) and is the main circulating vitamin D metabolite. [4] [5] Calcifediol has an elimination half-life of around 15 to 30 days. [4] [9] Calcifediol is further hydroxylated at the 1-alpha-position in the kidneys to form 1,25-(OH) 2 D 3, calcitriol.